std::integral_constant
Defined in header <type_traits>
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template< class T, T v > struct integral_constant; |
(since C++11) | |
std::integral_constant wraps a static constant of specified type. It is the base class for the C++ type traits.
Helper templatesA helper alias template
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(since C++17) |
Two typedefs for the common case where T
is bool are provided:
Defined in header
<type_traits> | |
Type | Definition |
true_type
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std::integral_constant<bool, true> |
false_type
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std::integral_constant<bool, false> |
Member types
Type | Definition |
value_type
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T
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type
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std::integral_constant<T,v> |
Member constants
Name | Value |
constexpr T value [static] |
static constant of type T with value v (public static member constant) |
Member functions
operator value_type |
returns the wrapped value (public member function) |
operator() (C++14) |
returns the wrapped value (public member function) |
std::integral_constant::operator value_type
constexpr operator value_type() const noexcept; |
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Conversion function. Returns the wrapped value.
std::integral_constant::operator()
constexpr value_type operator()() const noexcept; |
(since C++14) | |
Returns the wrapped value. This function enables std::integral_constant to serve as a source of compile-time function objects.
Possible implementation
template<class T, T v> struct integral_constant { static constexpr T value = v; typedef T value_type; typedef integral_constant type; // using injected-class-name constexpr operator value_type() const noexcept { return value; } constexpr value_type operator()() const noexcept { return value; } //since c++14 }; |
Example
Run this code
#include <iostream> #include <type_traits> int main() { typedef std::integral_constant<int, 2> two_t; typedef std::integral_constant<int, 4> four_t; // static_assert(std::is_same<two_t, four_t>::value, // "two_t and four_t are not equal!"); // error: static assertion failed: "two_t and four_t are not equal!" static_assert(two_t::value*2 == four_t::value, "2*2 != 4" ); enum class my_e { e1, e2 }; typedef std::integral_constant<my_e, my_e::e1> my_e_e1; typedef std::integral_constant<my_e, my_e::e2> my_e_e2; // static_assert(my_e_e1::value == my_e::e2, // "my_e_e1::value != my_e::e2"); // error: static assertion failed: "my_e_e1::value != my_e::e2" static_assert(std::is_same<my_e_e2, my_e_e2>::value, "my_e_e2 != my_e_e2"); }